J. Chem. SOC., Faraday Trans. I , 1982, '78, 3485-3492 Thermodynamics of a Silver, Silver Azide Electrode in Water and Water + Dioxan at Different Temperatures BY REBATI C. DAS,* MIHIR K. MISRA AND BATA K. NANDA Department of Chemistry, University College of Engineering, Burla 768 01 8, Orissa State, India Received 2 1 st December, 198 1 Ag, AgCl(s) I NaCl(m) : NaN,(m) I AgN,(s), Ag The e.m.f. of the cell in water and water + 10, + 20, + 30 and + 40 mass % dioxan has been measured at 5 OC intervals from 5 to 45 OC. The values of the standard potentials of the silver, silver azide electrode have been determined in water and the mixtures at these temperatures. The change in standard thermodynamic quantities (AG*, A H e and A@) for the electrode reaction have been evaluated. The thermodynamic parameters for the transfer of HN, from water to the mixtures have also been evaluated.The results are discussed in terms of the preferential solvation of ions. This paper is a part of our systematic work on secondary silver electrodes in water and water + dioxan mixtures at different temperat~res.~.~ Cell A was studied in water and water + 10, + 20, + 30 and + 40 mass % dioxan at temperatures between 278.15 an? 308.15 K and the changes in the thermodynamic quantities for the electrode reaction were calculated. (A) Ag, AgCW I NaCl(aq) (m) : NaN,(aq) (m) I AgN,(s), Ag EXPERIMENTAL Dioxan was purified as described previously.2 Sodium azide (GR) was purified by repeated crystallisation from an aqueous solution saturated at 90 "C by cooling it to 10 O C and adding an equal volume of alcohol.The resulting crystals were washed with acetone and the product was dried at room temperature. A freshly prepared solution of sodium azide was used in each experiment. Sodium chloride (GR) was recrystallised twice from water. Preparation of the silver, silver chloride electrode and cell solutions, the setting of the cells and the e.m.f. and conductivity measurements were essentially similar to those described previously.2 The silver, silver azide electrode was prepared essentially by the method of Taylor and Nims.* Platinum spirals coated with spongy silver were prepared as described previously.2 Silver azide was deposited by electrolysing a 0.1 mol dm-, solution of sodium azide for 20-30 min using a current of 5 mA with the silver bases as anodes and platinum wires as cathodes.The electrodes were then washed and kept in contact with water saturated with AgN, in order to age them. The electrodes were dipped in cell solutions for ca. 1-2 h before use. RESULTS AND DISCUSSION A summary of e.m.f. data for cell A at different temperatures is given in table 1. The e.m.f. of the cell is given by the equation + EJ E = Eg,,+klog--- mYN, my,,- 113 3485 FAR 13486 Ag, AgN, ELECTRODE I N WATER + DIOXAN TABLE I.--E-E, FOR CELL A FROM 278.15 TO 318.15 K IN WATER AND FROM 278.15 TO 308.15 K IN WATER -I- DIOXAN MIXTURES water T / K m/ 10-2 mol dm-3 278.15 283.15 288.15 293.15 298.15 303.15 308.15 318.15 0.5 1 .o 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 0.062 0 0.062 3 0.062 29 0.062 31 0.062 87 0.624 6 0.063 04 0.062 72 0.062 85 0.062 78 0.062 74 0.064 72 0.064 80 0.064 85 0.064 90 0.064 53 0.065 00 0.065 10 0.065 22 0.065 23 0.064 98 0.065 30 0.066 58 0.066 60 0.066 70 0.066 80 0.066 83 0.066 95 0.066 71 0.066 94 0.066 98 0.067 10 0.067 09 0.068 30 0.068 37 0.068 40 0.068 89 0.068 60 0.068 70 0.068 80 0.068 90 0.068 80 0.068 31 0.069 10 0.070 40 0.070 49 0.070 46 0.070 47 0.070 48 0.070 48 0.070 63 0.070 50 0.070 52 0.070 33 0.070 52 0.072 60 0.072 65 0.072 37 0.072 90 0.072 72 0.072 73 0.072 78 0.072 82 0.072 60 0.072 87 0.072 90 0.074 64 0.074 70 0.074 65 0.074 66 0.074 68 0.074 70 0.074 90 0.074 72 0.074 74 0.074 75 0.074 37 0.078 00 0.078 05 0.078 08 0.078 15 0.078 17 0.078 80 0.078 20 0.078 24 0.078 28 0.078 60 0.078 34 water + 10 % dioxan T/K m/ 1 0-2 mol dm-3 278.15 283.15 288.15 293.15 298.15 303.15 308.15 0.5 1 .o 2.0 3.0 4.0 5.0 6.0 8.0 9.0 10.0 0.068 10 0.068 14 0.068 14 0.068 40 0.068 50 0.068 46 0.068 58 0.068 59 0.068 75 0.068 84 0.070 10 0.070 25 0.070 23 0.070 32 0.070 34 0.070 40 0.070 44 0.070 92 0.070 12 0.070 60 0.072 40 0.072 50 0.072 30 0.072 53 0.072 56 0.072 60 0.072 68 0.072 70 0.073 06 0.073 35 0.074 50 0.074 65 0.074 40 0.074 68 0.074 76 0.074 88 0.075 02 0.075 03 0.075 12 0.075 17 0.076 30 0.076 45 0.076 32 0.076 36 0.076 42 0.076 31 0.076 51 0.076 76 0.076 66 0.076 70 0.078 30 0.078 45 0.078 50 0.078 60 0.078 74 0.078 77 0.078 83 0.078 93 0.079 40 0.079 30 0.080 20 0.080 30 0.080 10 0.080 35 0.080 40 0.080 48 0.080 55 0.080 68 0.080 72 - water + 20% dioxan T/K m/ 1 0-2 mol dm-3 278.15 283.15 288.15 293.15 298.15 303.15 308.15 0.5 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 0.075 20 0.075 30 0.075 12 0.075 37 0.075 37 0.075 41 0.075 43 0.075 50 0.075 52 0.075 73 0.077 70 0.077 80 0.077 74 0.077 97 0.077 99 0.078 13 0.078 21 0.078 30 0.078 37 0.078 49 0.078 35 0.078 48 0.078 30 0.078 60 0.078 71 0.078 85 0.078 94 0.079 02 0.079 12 0.079 30 0.081 00 0.081 10 0.081 22 0.081 31 0.081 51 0.081 53 0.081 56 0.081 66 0.081 82 0.081 92 0.082 40 0.082 50 0.082 64 0.082 70 0.082 92 0.082 98 0.083 02 0.083 08 0.083 31 0.083 37 0.084 38 0.084 40 0.084 43 0.084 62 0.084 64 0.084 66 0.084 71 0.084 69 0.084 80 0.084 88 0.086 40 0.086 50 0.086 54 0.086 57 0.086 70 0.086 78 0.086 88 0.086 89 0.086 94 0.086 973487 R. C.DAS, M. K. MISRA A N D B. K. N A N D A TABLE 1 .-(cont.) water + 30 % dioxan ~ ~ ~ _ _ _ _ _ _ _ _ ~ _____ T/K m/ 1 O2 mol dmP3 278.15 283.15 288.15 293.15 298.15 303.15 308.15 0.5 1 .o 2.0 3 .O 4.0 5.0 6.0 8.0 9.0 10.0 0.082 40 0.082 45 0.082 50 0.082 57 0.082 58 0.082 61 0.082 64 0.082 65 0.082 70 0.082 74 0.084 90 0.084 98 0.084 88 0.085 00 0.085 07 0.085 10 0.085 13 0.085 16 0.085 19 0.085 39 0.085 90 0.086 00 0.085 95 0.086 16 0.086 20 0.086 25 0.086 30 0.086 37 0.086 48 0.086 45 0.087 90 0.087 95 0.087 99 0.088 10 0.088 12 0.088 17 0.088 24 0.088 29 0.088 30 0.088 32 0.089 05 0.089 10 0.089 00 0.089 20 0.089 28 0.089 38 0.089 42 0.089 45 0.089 55 0.089 57 0.090 90 0.091 00 0.091 05 0.091 17 0.091 19 0.091 30 0.091 33 0.091 42 0.091 50 0.091 53 0.093 00 0.093 10 0.093 00 0.093 18 0.093 20 0.093 29 0.093 30 0.093 40 0.093 49 0.093 55 water + 40 % dioxan T/K m/102 mol dmP3 278.15 283.15 288.15 293.15 298.15 303.15 308.15 0.5 1 .o 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 0.090 00 0.090 10 0.090 22 0.090 35 0.090 43 0.090 54 0.090 85 0.090 76 0.090 86 0.091 00 0.092 40 0.092 50 0.092 63 0.092 70 0.092 80 0.092 82 0.092 93 0.093 00 0.093 20 0.093 34 0.092 90 0.092 95 0.092 97 0.093 00 0.093 06 0.093 18 0.093 10 0.093 36 0.093 42 - 0.095 00 0.095 08 0.095 19 0.095 14 0.095 35 0.095 42 0.095 46 0.095 52 0.095 68 - 0.095 95 0.096 00 0.096 11 0.096 17 0.096 20 0.096 24 0.096 30 0.096 32 0.096 40 - 0.097 90 0.097 95 0.098 01 0.098 07 0.098 16 0.098 18 0.098 28 0.098 35 0.098 42 - 0.099 40 0.099 45 0.099 51 0.099 59 0.099 63 0.099 67 0.099 70 0.099 80 0.100 00 - where k is 2.3026 RT/F and EaT is the liquid junction potential. Substituting the ~~ Azf.\/I Debye-Hiickel equation -1ogyi = ~ 1+.\/1+J we obtain = -+ bm.(3) All the terms in eqn (1)-(3) have their usual significance and b is (BN,-&l-). Extrapolation of the plot of E--EJ against m to m = 0 gives the value of Fell. Knowing the standard electrode potential of silver, silver chloride electrode,5* that of the silver, silver azide electrode can be calculated from the experimental value of Eqn (1) assumes that mcl- = mNy. This will be so if the two electrolytes (NaC1 and NaN,) are completely dissociated. We have observed in conductivity studies (unpublished) that both sodium chloride and sodium azide are almost completely dissociated in the range of concentrations up to 30 mass % dioxan.But in water + 40 :< dioxan there is some amount of association between the Na+ ion and the C1- or N; %ll- 113-23488 Ag, AgN, ELECTRODE IN WATER 4- DIOXAN ion. The error due to this is probably eliminated by our extrapolation procedure, because at rn -P 0 the electrolytes are completely dissociated. Furthermore, there is little uncertainty in the extrapolated values of cell because the plots of E - EJ against m were observed to be good straight lines. The values of EJ were calculated from equivalent conductivity values of the cell solutions of sodium azide and sodium chloride using Lewis and Sargent equation. The values of EJ varied between 0.001 77 and 0.00257 in water, 0.001 425 and 0.002956 in water+ lo%, dioxan, 0.001 301 and 0.002570 in water+20% dioxan, 0.001 190 and 0.002 798 in water + 30 % dioxan and 0.00 1 064 and 0.002 61 0 in water + 40% dioxan at all temperatures.The standard electrode potentials of silver, silver azide electrode (P) at all working temperatures and composition are shown in table 2. The Ee values can be represented as a function of temperature with a maximum deviation of 0.5 mV by the equation where t is the temperature in OC and a, b and c are empirical constants whose values are shown in table 3. Our E4 values of the silver, silver azide electrode in water can be compared with the values of an earlier determination by Taylor and N i r n ~ . ~ They used the same cell as we did (cell A), but also used cell B E@ = ~+b(t-25)+c(t-25)~ (4) Ag, AgCW I NaCl(aq) (4 II KCl(aq) I1 NaN,(aq) (m) I AgN,(s), Ag (B) with varying concentrations of the bridge solutions and they concluded that the liquid junction potential between the two equimolar solutions of sodium chloride and sodium azide in cell (A) is very small and thus neglected it.This is a clear approximation. In spite of that their values of Ee in water agree reasonably well with our values, as can be seen in table 4. Our determined values are more reliable because we have directly determined the liquid junction potentials instead of assuming them to be negligible. The standard free-energy, enthalpy and entropy changes ( A G e , A H 0 and A P ) for the reaction were calculated from the standard electrode potential values. These quantities are A G e = A + BT+ C T 2 represented by AgN3(s) + iH2(g) = + H+(aq) + N,(aq) ( 5 ) AH@ = A’+C‘T2 A S = D+B’T over the temperature range 278.15-308.15 K.In eqn (5)-(7), T is the temperature in K and the terms A , B, C, D , A’, B’ and C‘ are empirical parameters recorded in table 5. The change in the thermodynamic properties associated with transfer of 1 mole of HN, from water to the mixed solvent (AG?, AH? and ASP) are convenient quantities for the study of solvent e f f e ~ t . ~ These quantities were evaluated using the following relationships AGP = - F(,E$- ,EF) (8) where ,E$ and ,E$ are the standard electrode potentials in the mole-fraction scaleTABLE 2.-sTANDARD ELECTRODE POTENTIALSa (IN v) FOR THE SILVER, SILVER AZIDE ELECTRODE IN WATER AND IN DIOXAN+WATER FROM 278.15 TO 308.15 K T / K dioxan (%I 278.15 283.15 288.15 293.15 298.15 303.15 308.15 318.15 0 0.2963 ~0.0001 10 0.2930 f 0.0001 20 0.2919 f 0.0001 30 0.2885 f 0.0003 - + 0.0002 A 0.2957 f 0.0002 0.2927 f 0.0002 0.2906 f 0.0006 0.2868 k 0.0005 - + 0.0007 0.2949 If: 0.0002 0.2920 - + 0.0002 0.2891 f 0.0006 0.2849 f 0.0002 & 0.0004 0.2770 0.2940 f 0.0003 0.2912 - + 0.0002 0.2875 +0.0001 0.2828 - + 0.0002 0.2738 & 0.0002 0.2928 f 0.0001 0.2900 & 0.0001 0.2855 & 0.0001 0.2805 k 0.0004 0.2704 f 0.0003 0.2915 f 0.0002 0.2885 f 0.0001 0.2835 f 0.0001 0.2780 f 0.0002 0.2668 f 0.0001 0.2864 0.2900 - + 0.0002 If: 0.0002 0.2868 - f 0.000 1 - 0.28 12 - * 0.0002 - 0.2753 - f 0.0003 - 0.2629 - f 0.0003 - a The electrode potentials are expressed in the molal scale.The error limits were subjectively assessed considering the uncertainties in the e.m.f.readings (duplicate readings within & 0.4 mV) and the liquid junction potentials. w P 00 \D3490 Ag, AgN, ELECTRODE I N WATER + DIOXAN TABLE 3.-cONSTANTS OF EQN (4) dioxan (%I a 104b 10% 0 0.292 89 - 2.46 12 - 3.77 10 0.290 04 - 2.6246 - 5.66 20 0.285 58 - 3.9754 - 3.84 30 0.280 53 - 4.799 I - 3.99 40 0.270 47 - 7.0567 -4.41 TABLE 4.-cOMPARISON OF OUR VALUES FOR WITH THOSE OF NIMS AND TAYLOR4 278.15 0.2963 0.2959 288.15 0.2949 0.2942 298.15 0.2928 0.2919 308.15 0.2900 0.2889 318.15 0.2864 0.2854 for the electrode in water and in the mixed solvent, respectively. The thermodynamic transfer quantities were calculated from the standard electrode potentials in the mole-fraction scale because the effects of solvent on these thermodynamic quantities are more clearly reflected in this scale.’ A perusal of table 6 shows that the values of AH? and ASP are negative and increase with the increasing dioxan content, suggesting that the transfer is a structure-making process involving increased solvation of the ion.* It has been shown by Franks and Ives9 that the addition of a small quantity of an organic cosolvent to water enhances the three-dimensional hydrogen-bonded polymeric form of water.The present study of water+dioxan mixtures supports the view that for the transfer of HN, from water to highly structured mixtures of low dioxan content the net amount of order created by NH, in the mixed solvent is more than that in pure solvent. The changes in the thermodynamic quantities in the transfer process can further be separated into electrical and chemical parts where Q* = G e , H e or P.The electrostatic contribution to the AGP value can be calculated from Born’s equation and A p e , can similarly be calculated from the temperature coefficient of AGeel where D, and D, are the bulk dielectric constants and r , and r- are the radii of the cation and anion, respectively. The Y+ and Y- values are taken as 2.76lo9l1 and 1.17 respectively. Table 7 gives the chemical and electrical parts of the thermo- dynamic transfer quantities at 298.15 K.TABLE 5.-cONSTANTS OF EQN (5)-(7) dioxan (%) A B 104c D A’ i04B 104c 0 - 373 1.08 - 188.40 3559.74 180.43 - 4580.5 1 - 678 1.84 - 3395.60 - 5095.52 - 8754.20 - 3670.91 3860.95 172.32 - 6977.86 - 7262.16 - 3742.58 - 3822.50 10 12325.7 - 295.80 5385.64 285.21 10683.6 - 10310.6 20 - 5403.30 - 187.04 3780.62 225.68 - 5785.55 30 - 6565.65 - 183.90 40 - 93 16.01 - 180.64 4170.19 208.0 1 -11689.1 -91 36.20 TABLE 6.-TRANSFER THERMODYNAMIC QUANTITIES (MOLE-FRACTION SCALE) IN VARIOUS WATER + DIOXAN MIXTURES AT 298.15 K dioxan (%) AGy/kJ mol-l AH ?/kJ mol-l A@/J mol-1 K-l 10 -0.141 -0.917 - 2.605 20 -0.154 - 3.376 - 10.81 30 -0.173 - 5.753 - 18.72 40 + 0.263 - 11.13 - 38.213492 Ag, AgN, ELECTRODE I N WATER + DIOXAN PARAMETERS AT 298.15 K TABLE 7.-ELECTRICAL AND CHEMICAL CONTRIBUTIONS TO THERMODYNAMIC TRANSFER dioxan AGFe, AG&, AH& AH&,, A q e l AS& (%) /kJ mo1-l /kJ mol-' /kJ mol-1 /kJ mol-l /J mo1-I K-l/J mol-l K-l I0 - 0.564 0.423 0.842 - 1.760 4.717 - 7.322 20 - 1.268 1.114 0.475 -3.851 5.846 - 16.65 30 - 2.233 2.060 -0.225 - 5.529 6.735 - 25.45 40 - 3.599 3.863 - 1.317 -9.811 7.658 - 45.86 Further it is of interest to examine the primary medium effect which results from a difference of the ion-solvent interaction at infinite dilution in each so1vent.l39 l4 Thermodynamically it can be represented by (,* - s w 2k lim (logky,) = m + o where k is 2.3026 RT/F and the limit term indicates the primary medium effect.The activity coefficient ky refers to a value of unity for infinitely dilute solution with water as solvent. The values of lim, ~ (log $ y k ) at 298.15 K are given in table 8. This is a positive quantity increasing with increasing dioxan content. TABLE 8.-PRIMARY MEDIUM EFFECT OF N3 ION IN WATER+DIOXAN MEDIA AT 298.15 K 1 2 3 4 5 6 7 8 9 10 11 12 13 14 10 0.047 35 20 0.012 34 30 0.2080 40 0.3788 R. C. Das, G. Sahu and S. N. Mishra, Electrochim. Acta, 1974, 19, 887. R. C. Das, U. N. Dash and K. N. Panda, Electrochim. Acta, 1979, 24, 99. R. C. Das, U. N. Dash and K. N. Panda, Acta Chim. Acad. Sci. Hung., 1979, 99, 295. A. C. Taylor and L. F. Nims, J . Am. Chem. Soc., 1938, 60, 262. H. S. Harned and B. B. Owen, The Physical Chemistry of Electrolytic Solutions (Reinhold, London, 1967). P. K. Das and U. C. Mishra, Electrochim. Acta, 1977, 22, 59. Physico-chemical Processes in Mixed Aqueous Solvents, ed. F. Franks (Heinmann, London, 1969). H. S. Frank and M. W. Evans, J . Chem. Phys., 1945, 13, 507. F. Franks and D. J. G. Ives, Q. Retl., 1966, 20, 1. M. Paabo. R. G. Bates and R. A. Robinson, J. Phys. Chem., 1966,70, 247. R. N. Roy, M. Vernon, A. Bothwell and J. Gibbons, J. Electrochem. SOC., 1972, 119, 694. U. N. Dash, Fluid Phase Equilibria, 1981, 5, 323. R. A. Robinson and R. G. Stokes, Electrolytic Solutions (Butterworths, London, 1959). B. B. Owen, J . Am. Chem. Soc., 1932, 54, 1758. (PAPER 1/1972)